Method and apparatus for testing light-emitting device

ABSTRACT

Disclosed is a method for testing a light-emitting device comprising the steps of: providing a light-emitting device comprising a plurality of light-emitting diodes; driving the plurality of the light-emitting diodes with a current; generating an image of the light-emitting device; and determining a luminous intensity of each of the light-emitting diodes; wherein the magnitude of the current is determined such that the current density driving each of the light-emitting diodes is smaller than or equal to 300 mA/mm 2 .

REFERENCE TO RELATED APPLICATION

This application is a continuation application of U.S. patentapplication Ser. No. 13/365,820, entitled “METHOD AND APPARATUS FORTESTING LIGHT-EMITTING DEVICE”, filed on Feb. 3, 2012, now pending, theentire contents of which is incorporated herein by reference.

TECHNICAL FIELD

The application relates to a method for testing a light-emitting device.

DESCRIPTION OF BACKGROUND ART

As the technology of the light-emitting diodes (LEDs) develops, thelight-emitting diodes are applied widely. And now a light-emittingdevice adopting the light-emitting diode usually comprises more than onesingle light-emitting diode. For example, the light-emitting device suchas a High Voltage Light-Emitting Diode (HVLED), an Alternating CurrentLight-Emitting Diode (ACLED), or an Array that is commonly used as adisplay, a traffic sign, and a lighting, etc., comprises a plurality oflight-emitting diodes. Taking the HVLED as an example, as a singlelight-emitting diode works at a low voltage, the HVLED is formed by aplurality of light-emitting diodes connected in series. FIG. 1 shows anHVLED. The externally supplied AC voltage 11 is reduced in its voltagelevel by a converter 12 and converted into a DC voltage correspondingthereto. Then the converted DC voltage is fed into a plurality oflight-emitting diodes. As shown in FIG. 1, to form the HVLED, alight-emitting diode is connected with one another in series. And theneach series is connected in parallel with other series. This can be doneby a chip level design to layout the plurality of light-emitting diodesin a single chip, or by a package level to have the plurality oflight-emitting diodes in a package. However, some types of failurescaused by the manufacturing process or material defects, such as thecurrent leakage in a small local area or a surface stain/damage whichcauses the light shaded, cannot be detected by a conventional electricaltest. For a light-emitting device comprises a plurality oflight-emitting diodes connected in series and/or parallel, if only a fewlight-emitting diodes fail, it is difficult to be detected by aconventional electrical test. Besides, the uniformity of the luminousintensity of the plurality of light-emitting diodes which often concernsthe application cannot be measured in the conventional electrical testas well.

SUMMARY OF THE DISCLOSURE

Disclosed is a method for testing a light-emitting device comprising thesteps of: providing a light-emitting device comprising a plurality oflight-emitting diodes; driving the plurality of the light-emittingdiodes with a current; generating an image of the light-emitting device;and determining a luminous intensity of each of the light-emittingdiodes; wherein the magnitude of the current is determined such that thecurrent density driving each of the light-emitting diodes is smallerthan or equal to 300 mA/mm².

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a circuit diagram of a conventional HVLED.

FIG. 2 illustrates an apparatus for testing a light-emitting devicecomprising a plurality of light-emitting diodes in accordance with oneembodiment of the present application.

FIG. 3 illustrates an actual image of a light-emitting device capturedby a CCD in accordance with one embodiment of the present application.

FIG. 4A illustrates a method for testing a light-emitting devicecomprising a plurality of light-emitting diodes in accordance withanother embodiment of the present application.

FIG. 4B illustrates further steps which the method illustrated in FIG.4A may comprise in accordance with another embodiment of the presentapplication.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The light-emitting device to be tested in the present applicationcomprises a plurality of light-emitting diodes. The plurality oflight-emitting diodes may be formed in a series connection, a parallelconnection, or both series and parallel connection. The light-emittingdevice may be in a variety of forms. For example, the light-emittingdevice to be tested may be at a chip level (or wafer level) or a packagelevel. For the light-emitting device at a chip level, the light-emittingdevice can be a chip having one light-emitting diode, or a chipcomprises multiple light-emitting diodes monolithically integratedtogether. For the light-emitting device at a wafer level, thelight-emitting device is in a wafer form with a plurality oflight-emitting diodes, wherein the wafer can be separated later to formmultiple chips which each contains one or more light-emitting diodes bydicing. For the light-emitting device at a package level, thelight-emitting device can be one package containing one or more chipswhich are electrically connected together in such package, or multipleindividually packaged chips which are electrically connected to form thelight-emitting device.

FIG. 2 illustrates an apparatus for testing a light-emitting devicecomprising a plurality of light-emitting diodes in accordance with anembodiment of the present application. The apparatus 200 comprises acurrent source 210, an image receiving device 220, and an imageprocessing unit 230. The light-emitting device to be tested 201 (orDevice Under Test, herein after DUT 201) is placed under the imagereceiving device 220, and the current source 210 provides a current tothe DUT 201 for driving the plurality of the light-emitting diodes. Eachof the plurality of the light-emitting diodes emits a light in thedriven state. The image receiving device 220 receives an image of theDUT 201 in the driven state, and the image processing unit 230determines a luminous intensity of each of the light-emitting diodesaccording to the image received by the image receiving device 220.

The image receiving device 220 may comprises a microscope whichmagnifies the image of the DUT 201 when it is received. The imagereceiving device 220 may further comprise an image sensor 221, such as aCCD (Charge-coupled Device) or a CMOS image sensor to capture the imageof the DUT 201. The image sensor 221 may be placed abreast of aneyepiece 222 of the image receiving device 220, so that the imagereceived can be observed from the eyepiece 222 of the image receivingdevice 220 by an operator with his eyes and/or be capturedsimultaneously by the image sensor 221 with the signals of the imagebeing transferred to the image processing unit 230 for further processesor determination. When the image is observed from the eyepiece 222 ofthe image receiving device 220 by an operator with his eyes, theoperator determines the luminous intensity of each of the light-emittingdiodes. When the image is captured by the image sensor 221, the signalsof the image are transferred to the image processing unit 230. Afterfurther processes such as analog to digital conversion (ADC) by theimage processing unit 230, a gray level value for each of thelight-emitting diodes may be obtained to represent and be determined asthe luminous intensity of each of the light-emitting diodes. The graylevel is usually divided into a number of levels of an exponent of 2(i.e. 2^(n)). Generally, 256 (=2⁸) levels are used to represent the graylevel. The apparatus 200 may further comprise a comparing unit 231 tocompare the luminous intensity indicated by the gray level value of eachof the light-emitting diodes with a pre-determined luminous intensity todetermine whether the light-emitting diode is a defective light-emittingdiode or not. The pre-determined luminous intensity, for example, may bepre-determined from some statistic data, such as the average of theluminous intensities indicated by the gray level values of goodlight-emitting diodes. The image receiving device 220 may furthercomprises a filter 223 for filtering off a specific range of the wavelength of a light. This is useful when the difference between theluminous intensity of this defective light-emitting diode and theluminous intensity of other good light-emitting diodes is minor and notdiscriminable. The filter 223 filters off a specific range of the wavelength of a light and therefore makes the difference between theluminous intensity of this defective light-emitting diode and theluminous intensity of other good light-emitting diodes large enough tobe discriminable. The filter 223 may be set between the DUT 201 and theimage receiving device 220, or between the image receiving device 220and the image sensor 221 and the eyepiece 222. Furthermore, the imageprocessing unit 230 and the comparing unit 231 may be assembled in theautomated equipment 240 such as a computer. In addition, the currentsource 210 may be assembled in the same automated equipment 240 so theimage processing unit 230, the comparing unit 231, and the currentsource 210 may be controlled and coordinated for operation by, forexample, a computer program.

The current source 210 provides a current to the DUT 201 for driving theplurality of the light-emitting diodes, and the magnitude of the currentprovided can have a substantially constant value or a variant value.Each of the plurality of the light-emitting diodes emits a light in thedriven state. FIG. 3 shows an actual image of a light-emitting device asthe DUT 201 captured by the image sensor 221. In this example, the DUT201 comprises 16 light-emitting diodes arranged in 2 columns where eachcolumn has 8 light-emitting diodes. All the 16 light-emitting diodes areconnected in series. In one embodiment, the magnitude of the currentprovided by the current source 210 is increased with time from a smallvalue to a large value, for example, from 1 mA to 15 mA. As shown in thefigure, as the current provided increases, the luminous intensity ofeach of the light-emitting diodes becomes larger. It is noticed that one(the second one from the bottom in the left column, marked by anellipse) of the light-emitting diodes has less luminous intensitycompared with the others, especially when the current is from about 6 mAto about 11 mA. This indicates the darker light-emitting diode isdefective due to the current leakage. However, when the current providedis larger than 11 mA, i.e. from 12 mA to 15 mA, the difference of theluminous intensity between this defective light-emitting diode and theothers is not that obvious to differentiate. This is because the currentleakage is in a small and local area. As the current provided increases,both the leakage current and the operating current increase, but theincreasing rate of the leakage current is smaller than the increasingrate of the operating current. When the current provided is largeenough, for example, when the current is from 12 mA to 15 mA, theleakage current contributes only a small percentage of the operatingcurrent, and the difference of the luminous intensity between thedefective light-emitting diode and the others becomes minor and not sodiscriminable. In the above embodiment, the current sufficient toidentify the defective light-emitting diode is from about 6 mA to about10 mA. Taking the current of 10 mA as an example, if the area of the DUT201 is about 1 mm², the current density of each of the light-emittingdiodes in this light-emitting device is about 160 mA/mm² (i.e. 10 mA/(1mm²/16)). For most light-emitting devices, a current density smallerthan or equal to about 300 mA/mm² is small enough to differentiate thedefective light-emitting diode. In an alternative embodiment, thecurrent may be with a substantially constant value, and the currentdensity smaller than or equal to about 300 mA/mm² is small enough toscreen out the defect. In a conventional electrical test, the currentprovided to the light-emitting device is usually the operation currentof the light-emitting diode, and the leakage is usually not detected.Besides, even the current provided to the light-emitting device in aconventional electrical test is set to a small current such that thecurrent density of each of the light-emitting diodes is smaller than theoperation current density, because the current leakage normally happensin a small and local area and does not construe an open circuit, it isnot easy to detect defect like leakage in a conventional electricaltest. In contrast, with the luminous intensity of each of thelight-emitting diodes of the image, the defect like leakage can beeasily detected.

FIG. 4A illustrates a method for testing a light-emitting devicecomprising a plurality of light-emitting diodes in accordance withanother embodiment of the present application. The light-emitting deviceto be tested comprises a plurality of light-emitting diodes, and is asthose noted and illustrated above in the abovementioned.

The method may be carried out with the utilization of the apparatus aspreviously illustrated. The method comprises: providing a current todrive the plurality of the light-emitting diodes (step 401); providingan image receiving device (step 402); receiving an image of thelight-emitting device in the driven state by the image receiving device(step 403); and determining a luminous intensity of each of thelight-emitting diodes according to the image (step 404). In the step401, a current is provided to the light-emitting device for driving theplurality of the light-emitting diodes, wherein the magnitude of thecurrent has a substantially constant value or a variant value. Each ofthe plurality of the light-emitting diodes emits a light in the drivenstate. In one embodiment, the current provided has a variant value, andthe current is increased with time. In an alternative embodiment, thecurrent has a substantially constant value. In both embodiments, thecurrent density of each of the light-emitting diodes is smaller than orequal to about 300 mA/mm².

In the step 402, the image receiving device may be a microscope, and mayfurther comprise a filter for filtering off a specific range of the wavelength of light. In the step 403, an image of the light-emitting devicein the driven state is received by the image receiving device. In thestep 404, a luminous intensity of each of the light-emitting diodes inthe light-emitting device is determined according to the image by, forexample, an operator observing from the eyepiece of the image receivingdevice with his eyes. In one embodiment, the image receiving device mayfurther comprise an image sensor, such as a CCD or a CMOS image sensor,to capture the image of the light-emitting device. The image sensor maybe placed abreast of the eyepiece of the image receiving device so theimage received can be captured simultaneously by the image sensor, andthe signals of the image are transferred to an image processing unit forfurther processes or determination. That is, the image processing unitmay be assembled in an automated equipment, such as a personal computer,and in the step 404, the determining of the luminous intensity of eachof the light-emitting diodes according to the image may also beperformed by the automated equipment. After further processes of thesignals of the image by the image processing unit, such as analog todigital conversion (ADC), a gray level value for each of thelight-emitting diodes may be obtained to represent and be determined asthe luminous intensity of each of the light-emitting diodes. With themethod carried out by the automated equipment, it is easier to provide acurrent with a variant value to the light-emitting device, and thecurrent is increased from a small value to a large value with time andthe luminous intensity is correspondingly determined. These can beeasily carried out by the automated equipment with a computer programcomprising a loop, with the current being controlled to provide adifferent current value in the different execution of the loop.

As shown in FIG. 4B, the method may further comprise other steps. Forexample, the gray level value for each of the light-emitting diodes maybe optionally used to generate an intensity map to show the position ofeach of the light-emitting diodes and its corresponding luminousintensity, as shown in the step 405. The intensity map can also beoutputted to a computer monitor for the operator to compare the luminousintensity indicated by the gray level value with a pre-determinedluminous intensity to determine whether a light-emitting diode is adefective light-emitting diode or not, as shown in the step 406. Or asan alternative, in the step 406, the automated equipment may furthercomprise a comparing unit, and the comparing unit compares the luminousintensity indicated by the gray level value of each of thelight-emitting diodes with a pre-determined luminous intensity todetermine whether a light-emitting diode is a defective light-emittingdiode or not. The pre-determined luminous intensity can be collectedfrom statistic data, such as the average of the luminous intensitiesindicated by the gray level value of good light-emitting diodes.

Then, the method may further comprise the step 407 to compare the numberof defective light-emitting diodes with a pre-determined number forqualification. For example, normally only one or two defectivelight-emitting diodes in a light-emitting device is acceptable. In suchcase, the pre-determined number is set as 3. Similarly, this step 407can be performed by the operator or by the automated equipment with acomputer program. When the light-emitting device is at wafer level, thewhole wafer can be loaded to be tested. After testing, the method mayfurther comprises an optional step 408 to generate a qualificationstatus map showing the position of the tested regions in the wafer andits corresponding qualification status, wherein the qualification statuscomprises a qualified status or an unqualified status.

The method may further comprise the step 409 to mark the unqualifiedlight-emitting device. And when the luminous intensity is determinedautomatically by an automated equipment in step 404, step 406, and step407, a reconfirmation procedure is performed by the operator with eyesfrom the eyepiece of the image receiving device. The reconfirmationprocedure can also performed at the same time when the automatedequipment detects an unqualified light-emitting device and alarms andpauses to wait for the instruction from the operator, or thereconfirmation procedure can be made later after the whole wafer istested. Similarly, in the step 406, when the comparing is performed bythe automated equipment (the comparing unit) and a defectivelight-emitting diode is detected, a reconfirmation procedure may be setto be conducted by the operator in the same way illustrated as theabove. Besides, the intensity map generated in the step 405 and thequalification status map generated in the step 408 may be stored in theautomated equipment for the quality control or the engineering analysisin the future. In addition, the uniformity of the luminous intensity ofthe plurality of light-emitting diodes which often concerns theapplication but can not be measured in the conventional electrical testcan be obtained by this method. The intensity map generated in the step405 clearly shows the position of each of the light-emitting diodes andits corresponding luminous intensity indicated by the gray level value,and the uniformity may be calculated based on these data and bemonitored.

The foregoing description has been directed to the specific embodimentsof this invention. It will be apparent; however, that other alternativesand modifications may be made to the embodiments without escaping thespirit and scope of the application.

What is claimed is:
 1. A method for testing a light-emitting devicecomprising the steps of: providing a light-emitting device comprising aplurality of light-emitting diodes; driving the plurality of thelight-emitting diodes with a current; generating an image of thelight-emitting device; and determining a luminous intensity of each ofthe light-emitting diodes; wherein the magnitude of the current isdetermined such that the current density driving each of thelight-emitting diodes is smaller than or equal to 300 mA/mm².
 2. Themethod as claimed in claim 1, wherein the magnitude of the current is asubstantially constant value.
 3. The method as claimed in claim 1,wherein the light-emitting device comprises only two pads for thecurrent to be provided.
 4. The method as claimed in claim 1, wherein theluminous intensity is determined according to a gray level of the image.5. The method as claimed in claim 1, wherein the step of generating theimage of the light-emitting device comprises providing an imagereceiving device and receiving the image of the light-emitting devicefrom the image receiving device.
 6. The method as claimed in claim 5,wherein the image receiving device is a microscope.
 7. The method asclaimed in claim 5, wherein the image receiving device further comprisesan image sensor to capture the image of the light-emitting device. 8.The method as claimed in claim 5, wherein the image receiving devicefurther comprises a filter for filtering off a specific range of thewave length of light.
 9. The method as claimed in claim 1, wherein thestep of determining the luminous intensity of each of the light-emittingdiodes with the image is performed manually by a human being with eyes.10. The method as claimed in claim 1, wherein the step of determiningthe luminous intensity of each of the light-emitting diodes with theimage is performed automatically by an automated equipment.
 11. Themethod as claimed in claim 1, further comprising generating an intensitymap showing the position of each of the light-emitting diodes and itscorresponding luminous intensity.
 12. The method as claimed in claim 1,further comprising comparing the luminous intensity of each of thelight-emitting diodes with a pre-determined luminous intensity fordetermining whether each of the light-emitting diodes is a defectivelight-emitting diode or not.
 13. The method as claimed in claim 12,further comprising comparing the number of defective light-emittingdiodes with a pre-determined number.
 14. The method as claimed in claim1, wherein the light-emitting device is tested in a wafer form.
 15. Themethod as claimed in claim 14, further comprising generating anqualification status map showing the position of tested light-emittingdevice and its qualification status in the wafer, wherein thequalification status comprises a qualified status or an unqualifiedstatus.
 16. The method as claimed in claim 15, further comprisingmarking the unqualified light-emitting device.
 17. The method as claimedin claim 1, wherein the plurality of light-emitting diodes ismonolithically formed as a chip.
 18. The method as claimed in claim 1,wherein the plurality of light-emitting diodes is connected in series.